Why is the behavior called a fixed action pattern considered to be "fixed"?

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The animal will perform the behavior only at a certain time during its natural circadian rhythm.
Once the animal begins the behavior, it will complete it even if the stimulus that caused the behavior is removed.
The behavior is learned or "fixed" on the organism by observing her parents repeat the behavior many times while she was young.
The behavior is "fixed" in the animals brain by receiving a positive reward each time the behavior is done..